ECM/PCM Power Relay Sense Circuit Open
The P0688 code means the vehicle's onboard diagnostic system has detected: ECM/PCM Power Relay Sense Circuit Open. This is a critical severity code.
- Keep driving?
- No -- stop driving
- DIY difficulty
- hard
- Estimated cost
- $100 - $800 (parts and labor)
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Common Symptoms
- Check Engine Light on
- ECM cannot verify power relay status
- Vehicle may enter limp mode
- Possible no-start condition
Probable Causes (Ranked by Likelihood)
- Open in ECM power relay feedback circuit Very High
- Wiring damage or corrosion in the circuit High
- Poor electrical connection at module or component Moderate
- ECM or control module internal failure Low
Step-by-Step Diagnostic Procedure
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Use a scan tool to read data related to ECM/PCM Power Relay Sense
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Inspect all related wiring, connectors, and grounds for damage, corrosion, or loose connections
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Measure circuit voltage, resistance, and continuity to identify the fault type
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Check for technical service bulletins related to this code for your specific vehicle
Frequently Asked Questions
What does P0688 mean?
P0688 indicates a fault in the ecm/pcm power relay sense circuit open. This is an electronic control system code that affects the ECM's ability to manage the associated component or system.
Is P0688 a serious code?
Yes, this code indicates a critical control circuit issue that can affect vehicle operation and should be addressed immediately.
Can a low battery cause ECM-related codes?
Yes. Low battery voltage can cause various ECM and module communication codes. Always verify the battery is fully charged and the charging system is working before diagnosing electronic control module codes.
Sources
This page is built from documented references. Verify against your own service info before repair work.
P0688 on Diesel, EV & Equipment
The same code ID appears across other engines and platforms. The diagnostic flow varies by manufacturer — these are the platform-specific breakdowns:
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